1. Field of the Invention
The present invention relates to a propylene/ethylene-.alpha.-olefin block copolymer which is superior in impact resistance and processability. In the present invention, the propylene/ethylene-.alpha.-olefin block copolymer means a polymer obtained by successively synthesizing a propylene polymer component and an ethylene-.alpha.-olefin copolymer component. However, this block copolymer is not a true block copolymer with which said components (moieties) are perfectly chemically bonded each other, but is a kind of polymer compositions.
2. Description of the Related Arts
Polypropylene has widely been used for various purposes because of excellent physical properties. For example, polypropylene has recently been used for various purposes such as automobile interior/exterior parts, electrical parts, etc. because of excellent rigidity, heat resistance, gloss and moldability as well as low price. However, there was a problem that polypropylene is insufficient in impact resistance because of the crystallizability and, therefore, the use thereof is limited.
In order to improve the impact resistance of propylene, a process of polymerizing propylene with ethylene or the other olefin stepwise to form a block copolymer has hitherto been used. In the production of a block copolymer of conventional stereoregular polypropylene, a titanium trichloride catalyst or a titanium-magnesium composite catalyst is exclusively used. These catalysts normally have low randomness in the copolymerization reaction and wide composition distribution. Therefore, even if a rubber-like copolymer is produced at the second-stage after crystalline polypropylene was produced at the first-stage, the impact resistance is sometimes insufficient in a specific use.
As a method of improving the non-homogeneity of the rubber-like copolymer moiety, there is disclosed a method of producing a propylene/ethylene-propylene block copolymer or a block copolymer of propylene/propylene and at least one of ethylene and an .alpha.-olefin having 4 to 20 carbon atoms, using an uniform catalyst such as metallocene, which produces a homogeneous copolymer having large randomness (Japanese Patent Kokai Nos. 5-202152 and 6-172414). However, when using the above copolymer as the rubber-like copolymer moiety, the impact resistance is improved but the rigidity as an intrinsic characteristic of polypropylene is liable to be drastically deteriorated.
On the other hand, as a method of maintaining the rigidity to high level with improving the impact resistance of polypropylene, there has been used a method of blending polypropylene with a rubber-like substance having large randomness and narrow composition distribution, such as ethylene-propylene copolymer rubber (EPR), ethylene-butene copolymer rubber (EBR), ethylene-propylene-diene copolymer rubber (EPDM), etc. Although these rubber-like substances are normally produced with a vanadium catalyst system, there has recently been disclosed an improvement accomplished by blending an ethylene-higher .xi.-olefin copolymer rubber (.alpha.-olefin having 4 to 8 carbon atoms) with an uniform catalyst system such as metallocene catalyst system (Japanese Patent Kokai Nos. 6-192500, 6-192506, 6-248156 and 7-102126). However, there arises a problem in blending operation that such a rubber-like copolymer can not be used after pelletizing, like a crystalline resin, because the shape thereof is not easily handled according to the composition. Besides, with respect to a method of blending such a rubber-like copolymer, an operation of blending is required and a kinds of the rubber-like copolymers is expensive.
There is also disclosed a technique about a thermoplastic elastomer which is superior in moldability by means of olefin block copolymerization (Japanese Patent Kokai (laid-open) Nos. 57-61012, 58-32616, 58-71910, 58-103548, 59-105008 and 1-297408). In the production of these olefin block copolymers, a titanium trichloride catalyst system or a titanium-magnesium composite catalyst system is mainly used. However, the mechanical properties such as impact resistance is sometimes insufficient in a specific use. Furthermore, when using the catalyst, relative reactivity of .alpha.-olefin to ethylene tends to be markedly lowered depending upon the increase of the number of carbon atoms of the .alpha.-olefin in view of the property of the catalyst.